In passing through the human body, gamma photons have a certain probability of scattering due to the Compton effect. Such scattering changes the direction and energy of the photons. When a photon that has been scattered is detected by the gamma camera, false position information is derived from the scattered photons. Thus, the scattered photons cause events that are unwanted for use in constructing the image. Other unwanted events exist. For example, the radiation emitted from the patient often excites lead (K) X-rays from the collimator and other lead parts. These X-rays also impinge on the detector and may be registered as events. These X-ray photons constitute an additional source of image blurring.
The problem of X-ray induced events arises especially for radio isotopes emitting photons in the energy range of 88-120 KEV. In this range, the lead X-ray excitation probability is high and the spectrum of these photons coincides with a relevant part of the isotopes spectrum, by partially overlapping the photopeak. Thus, the unwanted part of the spectrum in each pixel has two terms: one made up of the Compton scattered photons, and the other made up of the lead X-ray photons.
In principle the events caused by unwanted photons should be discarded. However, it is not easy to arrive at criteria that are efficient and effective for discarding such events. For example, an energy level criterion is not effective because although the photon loses part of its energy in the scattering process, the energy resolution of the typical gamma camera is such that there is a large amount of overlap between the energy of unscattered and scattered photons.
The invention of the previously mentioned patent application provided methods and means for qualitatively and quantitatively improving the recorded images by significantly reducing the contribution of Compton scattered photons to the final image to thereby providing a practically Compton-free image within seconds after acquisition. The invention accomplishes the task of reducing the number of events caused by Compton scattered photons by locally determining the energy spectrum and fitting the determined energy spectrum with a "trial" function composed of a photopeak component of known energy shape but unknown magnitude and a Compton scatter component having a theoretically derived energy shape and an unknown magnitude for each pixel of the image.
The true physical characteristics of the Compton process are used in the previously mentioned patent application to derive Compton multi-scatter functions which are subsequently used to construct the Compton scatter component energy spectra. Thus, the previous patent application uses the following inputs to determine the unknowns; (i.e., the magnitude of the photopeak component and the magnitude of the Compton multi-scatter components):
1. the measured energy spectrum per pixel. This includes counts due to scattered and unscattered photons, and PA1 2. the measured system energy spread function for the isotope centerline which provide the photopeak energy shape.
The shape of the Compton component of the trial function is analytically derived in the prior application by converting the Nishina-Klein Equation that describes the physical relativistic scattering of photons with electrons into a probability distribution for a photon to scatter from a given energy to a lower energy in a single interaction with an electron. Repeated convolutions are used to obtain the probability distribution for the higher order scatter terms.
By locally fitting the trial function to the measured energy spectrum of acquired data, the values of the multi-scattered Compton co-efficients and the photopeak magnitude were obtained. This enables the removal of Compton contamination from the acquired data.
The prior invention, however, assumed a single photopeak. In certain isotopes there is more than one photopeak. If a single peak is assumed when more than one peak actually exists, the removal of scattered events from the image will be incomplete.
Accordingly, the invention of this application is an improvement over the invention of the prior mentioned applications in that, among other things, it takes into account radio isotopes having more than one peak and also takes into account all unwanted events due to Compton scattered photons and photons derived from such phenomena as X-rays caused by gamma radiation interacting with lead components.